There is a D natural modifier at the begining here. Does that mean that all of the D's in the bass clef are natural or just this specific one?
That 'modifier' (accidental) affects the note concerned until the end of the bar it's in. The very next barline cancels any or all accidentals in that bar. At which point the original key signature is obeyed.
It also does not make any other note change. As in, if there was another D in a different octave, that would itself need its own accidental to be present.
'Accidental' is rather an unfortunate term - as it's there on purpose! Perhaps 'temporary' would be more appropriate!
Should there be need for that D note to be D♮ in any other bar, including the very next one, it would have to be written in again.
The accidentals (which is the proper name for the "modifier" you are referring to) are only applicable for the measure in which they appear and they override the key signature. After the end of the measure every D you'll meet with be back to D# (from the key signature) unless an accidental is used again.
So in your example, in the 42nd measure you have a D natural, but on measures 43, 45 and 46 you have a D#.
In order to avoid confusion, it's common to use something called courtesy accidentals, which is an accidental in a parenthesis to remind you the accidental after the end of the measure like this: